The SiC single crystal is now drawing attention as a material for power semiconductor application, and a higher-quality substrate is required for practical use of the SiC single crystal.
A high quality crystal, i.e., a crystal having few dislocations, must be used as a seed crystal in order to produce a high-quality grown crystal. However, when quality of the seed crystal is improved, and when density of screw dislocations is particularly decreased thereby, step supply sources, which are necessary for inheriting the polytype of the seed crystal, are decreased. Consequently, heterogeneous polytypes are disadvantageously readily formed. In a proposed technique as described in Patent Literature 1, a screw dislocation formation region is provided in part of a surface of the seed crystal, and crystal growth is performed such that a {0001}-plane (c-plane) facet is superimposed on the screw dislocation formation region. This enables growth of a single crystal while suppressing formation of the heterogeneous polytypes.
Unfortunately, an SiC single crystal having a larger diameter is recently demanded, and the diameter of a grown crystal has been accordingly increased. Along with this, it has been clarified that the heterogeneous polytypes may not be stably suppressed by the above-described technique.
Through many experiments, we have discovered that formation of the heterogeneous polytypes is deeply related not only to the superimposition of the c-plane facet over the screw dislocation formation region as considered in the past, but also to the shape or size of the c-plane facet in an initial stage of crystal growth as a more important factor. In particular, the following is now clarified. That is, as the seed crystal has a larger diameter, the c-plane facet in the initial stage of crystal growth may have a more unstable shape such as an elongated linear shape rather than a stable small-circle-shape as in the process of crystal growth, and the heterogeneous polytypes are readily formed due to such an unstable shape.
As a reason why the heterogeneous polytypes are readily formed due to the elongated c-plane facet, step supply from a screw dislocation on the c-plane facet is possibly not sufficiently distributed over the entire c-plane facet. Hence, a screw dislocation formation region, which allows screw dislocations to be densely formed over the entire formation region of the c-plane facet, must be formed. This causes degradation in quality of the crystal as a whole. Moreover, the c-plane facet is likely to be separated by slight fluctuation of temperature of a growth plane or slight fluctuation of sublimated gas concentration on the growth plane. This leads to formation of a region, in which collision between steps occurs, in a growth plane other than the facet. In such a case, the heterogeneous polytypes are readily formed from a low-quality portion caused by the collision between steps.
As well known, the c-plane facet is formed in the neighborhood of a {0001} plane that is located on crystallographically higher position than its periphery in a crystal surface. It is therefore clear that the c-plane facet in the initial stage of crystal growth is affected by surface morphology of the seed crystal.
Various proposals have been made on the surface morphology of the seed crystal in the past.
For example, Patent Literature 2 discloses a technique where a conical seed crystal, of which the central axis direction is within plus or minus 10 degrees from the <0001> direction and the vertical angle is 20 to 90 degrees, is used in order to reduce micropipes and screw dislocations in a grown crystal.
Although such a pointed seed crystal must have a height of about 100 mm for a seed crystal having a diameter of 6 inches (152.4 mm), such a crystal is less likely to be produced. In the seed crystal having such a level difference, it may be difficult to adjust a growth rate of each of portions near the top and the bottom of the seed crystal, resulting in sublimation of the top during crystal growth, and consequently the shape of the seed crystal may not be maintained. If the shape of the seed crystal is maintained, the seed crystal having such a shape enables a dotted c-plane facet to be formed on the apex of the seed crystal in an initial state of crystal growth. Unfortunately, since the growth direction is similar to an a-axis direction particularly in the initial state of crystal growth, a growth region has no screw dislocation or has an extremely small density of screw dislocations as mentioned as an advantageous effect in Patent Literature 2. As a result, screw dislocations in the c-plane facet formed in the initial state of crystal growth are exhausted, resulting in formation of heterogeneous polytypes.
Patent Literature 3 discloses a technique where crystal growth is repeatedly performed with a growth plane provided with an offset angle of 20 degrees or more from a {0001} plane.
In Patent Literature 3, the offset angle of the growth plane is large, i.e., at least 20 degrees; hence, a screw dislocation contained in a seed crystal is easily converted into a dislocation in a basal plane. As a result, screw dislocations in the c-plane facet formed in the initial state of crystal growth are also exhausted, resulting in formation of heterogeneous polytypes. Moreover, Patent Literature 3 exclusively shows an Example where a {0001}-plane uppermost portion is formed by two inclined planes or one inclined plane and a side face. In this case, the most-upstream portion of the offset direction has a linear shape (corresponding to an intersection line between the inclined planes or an intersection line between the inclined plane and the side face), and a c-plane facet also has a linear shape. As a result, the c-plane facet shape easily becomes unstable, and thus heterogeneous polytypes are likely to be formed.
Patent Literature 1 describes a technique where one or a plurality of inclined planes is/are provided on a surface of a seed crystal to control a formation position of a c-plane facet.
In the technique described in Patent Literature 1, an offset angle of a growth plane is relatively small at the most-upstream portion of the offset direction on which the c-plane facet is formed, and thus a screw dislocation is allowed to exist. Patent Literature 1 further describes that a plurality of inclined planes having different inclination angles or inclination directions are provided to form a corner in the upstream portion of the offset direction, so that a c-plane facet position is controlled in the process of crystal growth.
However, as described in Patent Literature 1, even if a dotted {0001}-plane uppermost portion is formed by the plurality of planes to control the formation position of the c-plane facet, heterogeneous polytypes are more likely to be formed with an increase in diameter of the seed crystal. After the crystal growth, the grown crystal has been sliced to investigate formation of the c-plane facet in the initial stage of crystal growth. As a result, it is found that the c-plane facet is formed not only on the {0001}-plane uppermost portion but also on a ridge line between the planes. Consequently, the shape of the c-plane facet cannot be sufficiently controlled only by forming the dotted {0001}-plane uppermost portion by the plurality of planes.
Patent Literature 4 proposes a technique where a seed crystal, in which the shape of a growth plane is processed such that an offset angle of the growth plane is decreased along a direction from a {0001}-plane lower portion to a {0001}-plane uppermost portion on the growth plane, is used to prevent a dislocation flow from an offset upstream portion into an offset downstream portion.
However, even if the technique described in Patent Literature 4 is used, heterogeneous polytypes may be formed. A grown crystal having the heterogeneous polytypes has been sliced to investigate the shape of the c-plane facet in the initial stage of crystal growth. As a result, it is found that the c-plane facet having an unstable shape (a linear shape) is formed. Consequently, it is considered that the shape of the c-plane facet formed in the initial stage of crystal growth cannot be sufficiently controlled only by decreasing the offset angle at the offset upstream portion as described in Patent Literature 4.